The cold extrusion of gears, such as externally toothed pinion gears and internally toothed ring gears for vehicular automatic transmissions is an extremely advantageous gear manufacturing process. The process provides a precise forging requiring only minimal trim and finished grind operations to produce a finished part. However, cold extrusion does require that significant extrusion forces be applied against the forging blank to cause cold flow of the material. Thus, a forging press having a high tonnage rated capacity is required. Further, the forces on the die teeth are significantly high, thereby creating high residual stresses which can contribute to the breakage of the die teeth, or other premature failure. High frictional forces are also a concern, i.e., the frictional force between the blank being forged and the inner or outer die pieces.
When such gears are provided with helical gear teeth, bending stress on the die teeth is also of concern. Bending stress is caused by the fact that the metal of the gear blank is caused to change direction from pure axial flow (i.e., coaxially with the press anvil such as with spur gears) to a combined axial and helical direction of flow typical of helix gears. The bending stress is greatest at the forward end of the die tooth. Various designs for the die teeth have been proposed to minimize bending stress in this area, such as shown in U.S. Pat. Nos. 4,622,842 and 5,052,210 assigned to the assignee of the present invention.
It is also known to reduce the stress on the die teeth by eliminating or substantially reducing the friction between the gear blank and the non-toothed die part. Such a technique is shown and described in U.S. Pat. No. 4,878,370, also assigned to the assignee of the present invention, the teachings of which are incorporated herein by reference. Therein, there is shown the forging of an internally toothed helical ring gear wherein the annular outer die ring is axially spring-loaded, and thereby allowed to move with the workpiece at a maximum rate equalling that of the punch as the workpiece is extruded through the length of the mandrel. Friction between the workpiece is further reduced by utilizing a free-floating, self-centering mandrel, and by providing the mandrel die teeth with a tapered relief portion rearward of the leading end of the die teeth.
Broaching is also a common technique for producing gears, particularly ring gears, i.e. by cutting the teeth to near finished or finished dimension. The commonly used single axially reciprocating broach may include an axially incremental tooth configuration, in sections, such that at each section more and more of the desired finished tooth profile is cut by the broach. Typically, for a helical ring gear having a six inch internal diameter, the broach is in the order of ten to twelve feet long. It is made of high grade steel and extremely expensive to fabricate. It does produce a precision part, and the finishing operations are less than that of the cold extrusion technique described above.
Further with the broaching of helical gear teeth, it is common to allow the broach and/or the workpiece to rotate about the axis of the broach (and annular outer die part) to allow cutting the teeth on a helix.